The present invention relates generally to a measurement apparatus for a stepping motor, and more specifically to an apparatus for measuring load torque, load inertia moment, output or efficiency, and backlash using a stepping motor.
Stepping motors find very wide applications in various fields as mechanical driving sources because their rotary angle and rotary speed can be controlled very accurately using digital pulses under control of a micro computer. For example, stepping motors are being used extensively in booming OA (office automation) equipment such as floppy disk drives (FDDs), hard disk drives (HDDs), printers, electronic typewriters, plain paper copy (PPC) machines, recorders, plotters, etc. and FA (factory automation) machines and equipment such as industrial robots, etc.
Load torque, load inertia moment, output or efficiency and backlash are major characteristics of stepping motors which affect development, design, production and quality control of stepping motors. Unfortunately, it is very difficult to measure such characteristics or performance with a high degree of accuracy.
For example, in measuring the load torque of a stepping motor using a spur reduction gear coupled to the mechanical component, the torque tends to vary over a wide range. That is, the load torque may be, for example, 100 g.multidot.cm if the intermeshing between the pinion gear mounted on the motor output shaft and the first stage gear is relatively loose but 300 g.multidot.cm if the above intermeshing is hard (or pushed hard). Also, the intermeshing of the subsequent gear stages affects the measurements with less influence as the stage becomes farther from the first stage. Additional influential factors include the degree of eccentricity and precision between shafts of the mechanical components, and accuracy of assembling the mechanical components such as parallelism, tension of timing belts and wires, etc.
A conventional method of measuring the torque of a stepping motor (the motor torque required by the load) is to mount a dummy motor to the system to be measured (mechanical component). The dummy motor is equal in size and shape to the actual motor but excluding coils, magnets, etc. A torque gauge is coupled to the motor output shaft in the motor pinion by way of a collet chuck or the like. The torque is measured by manually rotating the motor. Another method of measuring the torque is to use a specially designed motor having the shaft extending from both sides of the motor. The motor has the same specification as the actual stepping motor. A torque gauge is coupled to the torque measurement shaft of the motor using a collet chuck while coupling the mechanical components to the other side of the motor shaft.
Another method of obtaining the load torque T (g.multidot.cm) comprises mounting a pulley of radius r (cm) on the motor output shaft and winding a string around the pulley to measure the force F (g) using a spring scale. The torque T is determined by the following expression: EQU T=F.times.r (g.multidot.cm)
Still other methods of measuring torque include the use of a torque meter utilizing the current/torque characteristic of a DC motor and the use of a torque gauge between the motor and the load.
As mentioned above, there are many methods of measuring the torque of a motor. However, none of them is satisfactory especially in regard to measurement accuracy.
For example, in the method of using a dummy motor or a pulley and a spring scale it is difficult to provide a desired number of revolutions to the load. In the former method, it is difficult to avoid the increase in load torque due to side loading by a slanted torque gauge. In the latter method, it is also difficult to stably mount the pulley and thus to avoid the effect of side load.
In the method of using a stepping motor having the same specification as the actual stepping motor, there is a magnet in the rotor of the motor. This means that the detention torque of the magnet is unavoidably included in the measurements. When the rotor is rotated, the magnet will generate electricity in the stator coil, thereby causing dynamic braking. Also, inclination of the torque gauge will present a side load to the motor bearing in such a manner as to increase the load torque. These are causes for reduced measurement accuracy.
In the method of using the current/torque characteristic of a DC motor, it is required to disassemble the motor whenever the measurement is to be made. The load torque tends to change due to intermeshing of the pinion and gear when reassembling the motor. It is therefore difficult to know exactly how large a torque is generated by the motor in actual operation.
The above discussions lead to the conclusion that in the conventional methods for measuring torque it is impossible to measure the torque of a motor coupled to the load with a high degree of accuracy. In other words, what is needed is to know the load torque of an individual motor in quantity production with fluctuations of intermeshing of the pinion gear and the first stage gear and tension of the motor timing pulley and the timing belt. However, what is actually done is to guess the load torque by substitution means different from the actual structure. Accordingly, it has been impossible to know the torque margin or the torque generated by the motor and the load torque. The practice in production is to guarantee product reliability by performing time consuming and expensive voltage fluctuation tests, temperature tests, aging tests, printing tests, etc. before product shipment. However, when the mechanical components do not operate properly, it was necessary to spend a long time investigating the cause of trouble which may have been attributable to insufficient torque margin by designer's mistake, improper adjustment in the production line, insufficient torque due to trouble in the motor, troubles in the mechanical components, etc.
There are similar problems in measuring load inertia moment, output or efficiency and backlash of a stepping motor as a result of the above difficulties associated with the torque measurement.